[OSM-talk] How to georeference OSM Mapnik tiles?

[Rahkonen Jukka]

> Lähettäjä: Martijn van Oosterhout [mailto:kleptog at gmail.com] 
> <Jukka.Rahkonen at mmmtike.fi> wrote:
> > I would like to convert some OSM tiles to geotiff format, 
> but I wonder 
> > how can I get the georeferencing information I need for it. What I 
> > would need to know is:
> > - Which projection the tiles are in (EPSG code or projection 
> > parameters)
> 
> It's not as official projection, though people refer to it as 
> EPSG:90013. It's also known as spherical mercator, the same 
> as google uses. The usual definition is:
> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 
> +x_0=0.0 y_0=0 
> ++k=1.0 +units=m +nadgrids=@null +no_defs

Thanks, that's exactly what I needed.

> > - Pixel size for each zoom level
> Zoom 0 is 256x256, each zoomlevel down is twice as big as the 
> previous in each dimension

Sorry for being unclear, I meant to ask "What are the dimensions of a single pixel (width and heighth) in geographical units in the real world?" The unit seem to be meter according to the definition above.

> > - What is the anchor point of the tile and its corresponding 
> > coordinates
> Zoom 0 has 1x1 tiles, Each zoomlevel down has twice as many 
> in each direction

Here I meant to ask what is the most natural point to use as reference point, for example the top-left corner of top-left pixel of each tile. But I think I will be able to solve this by the reference you sent.

-Jukka Rahkonen-